The present invention relates to polyurethane contact lenses.
Contact lenses have been manufactured by a variety of methods, including lathing, and cast molding. Lathing is not able to meet the demands of cheap, high-volume, and fast production. Efforts to reduce the inherent cost disadvantages of lathing have produced a process that is a hybrid of lathing and cast molding. For example, lenses may be prepared by casting one side of the lens and lathing the other side. This process is cheaper than lathing, but not as cheap as a complete cast molding process.
Cast molding requires the use of two complementary molds. These molds are often disposable, and the cost to replace the mold for each new lens is a significant part of the total cost of the final lens. Furthermore, lenses made by cast molding also suffer a large number of quality defects during in situ polymerization, due to shrinkage. For example, shrinkage may cause surface voids and the non-adherence of the final product to the lens design. Others have attempted to eliminate shrinkage and thereby improve cast molding techniques.
In the contact lens industry, attempts have been made to use injection molding processes to make contact lenses from polymethylmethacrylate (PMMA). PMMA lenses are hard and not oxygen permeable, i.e., they do not compare to the quality of hydrogel lenses. Thus, while injection molded processes, such as typically used in the plastics industry, are capable of high-speed, high-volume, consistent-quality mass production, there have not been good contact lens materials that could take advantage of those plastics manufacturing processes.
A common type of contact lens is made from a hydrogel, typically a crosslinked hydroxyethyl methacrylate polymer or copolymer, containing 37-75% water. These lenses are soft gels, that have good oxygen transport properties, and are generally known as “soft lenses”. These lenses are very comfortable for the wearer, but do not have ideal optical properties. Other lenses have been made from silicone rubber, also a soft lens material; or silicone or fluorine containing acrylate or methacrylate polymers, forming a rigid gas permeable (RPG) lens. These lenses have a hydrophobic surface, reducing comfort to the wearer, and may also be difficult to manufacture.
Polyurethane polymers and copolymers have also been used for contact lenses. Examples are described in U.S. Pat. Nos. 3,786,034; 4,255,550; 4,359,558; 4,454,309; 4,886,866; 5,039,458 and 5,674,942; as well as Japanese Patent publication 11255859; PCT international publication nos. WO 98/42497 and WO 98/00205. Some of these describe polyurethane hydrogels. In addition, polyurethane hydrogels are also described in U.S. Pat. Nos. 4,118,354; 4,644,033; 4,668,564; and 5,354,835.
There is a need for a polyurethane hydrogel for reaction molding contact lenses wherein reaction injection molding offers the greatest economic benefits. These lenses should be soft, have wettable surfaces, high oxygen transmission properties, and high physical strength. Furthermore, the lenses should be able to be sterilized by autoclaving or chemicals.